Manufacture of a silver halide photographic light-sensitive material usually comprises preparation of photographic compositions and treating processes thereof as described below. As a hydrophilic colloid used in a light-sensitive material, gelatin is mainly used at the present time.
(1) 1st ripening: formation of a silver halide colloidal composition called a photographic emulsion commonly, which is comprised of a gelatin sol containing silver halide grains suspended in it. PA1 (2) Desalting. PA1 (3) 2nd ripening: chemical ripening. PA1 (4) Preparation of a coating solution. PA1 (5) Coating, cooling to a gel and drying: the coating solution sol prepared is coated on a support, cooled to set and dried to a xerogel.
A photographic coating solution is prepared by adding additives necessary for proper photographic properties to a photographic emulsion, which has undergone the 2nd ripening, and adjusting its solution properties such as concentration and viscosity required in coating. In the addition of oil-soluble additives such as couplers or development inhibitor releasing compounds (DIR) to a color light-sensitive material, these are generally dissolved first in a high boiling solvent and dispersed in a hydrophilic colloid, then the dispersion prepared is added to a photographic emulsion.
In the industrial production of light-sensitive materials, a photographic emulsion and the above composition related thereto are preserved for a given period of time in the above manufacturing processes for reasons of factory operation and quality control of products. Such preservation is generally applied to photographic compositions such as a dispersion, a photographic emulsion after desalting or the 2nd ripening, and a coating solution freshly prepared for use.
Among these steps, preservation of a coating solution has advantages (1) that it makes possible to examine a coating solution itself in the course of preservation, and thereby the solution's photographic properties can be confirmed before coating, (2) that deterioration in photographic properties of a coating solution due to a prolonged standing can be prevented because only a dissolving process is needed as the preliminary arrangement for coating, this lessens the load in operation and installation, and (3) that a large amount of a uniform coating solution can be prepared at a time only by blending preserved emulsions. Similar advantages can also be brought out by preservation of a photographic emulsion or a dispersion, and requirements for an improved productivity and a high functional reliability can be met with the enlargement of manufacturing scale and rise in coating speed of light-sensitive materials.
However, there often arise various problems which impair functions of those compositions during the preservation or by sol-gel transformation. The preservation of these compositions has so far been usually practiced by chilling or freezing them in order to avoid the deterioration attributable to the progress of chemical reactions or the propagation of microorganisms caused during the preservation.
Such low-temperature preserved photographic compositions are each dissolved and blended by a necessary amount at the time of adjusting conditions of a coating solution, but these photographic compositions have usually undergone many changes of state up to that time.
That is, a photographic composition is in a sol state when prepared freshly, and then it undergoes changes of state such as gellation (setting to jelly) by low temperature preservation, dehydration and transformation into coagel (cryohydrate gel) by refrigeration and freezing, gellation through thawing, transformation into a sol by dissolution on heating, gellation by coating and cooling, and transforming into a xerogel by drying. While the state changes in succession as mentioned above, the composition is subjected to severe changes such as syneresis and dispersion of the contents and aggregation of the suspended particles due to the approach and contact thereof, as the structure or volume of gelatin micells changes. As a result, functions possessed by the composition before the preservation cannot be fully recovered in many cases.
Such thawing and dissolving of the composition has so far been practiced by placing it in a warm dissolving water, followed by stirring. But Japanese Pat. O.P.I. Pub. No. 193134/1990 recommends natural thawing in a refrigerator of 0.degree. to 7.degree. C. In any case, however, a prolonged preservation under a melting condition at above 0.degree. C. gradually deteriorates properties of the composition, and natural thawing over a long time spoils the readiness in production engineering.
As techniques to melt a preserved photographic composition gel from the standpoint of production engineering, Japanese Pat. O.P.I. Pub. No. 100439/1988 proposes a method which comprises the steps of placing a composition gel in a melting tank, stirring it while continuing conductive heating from the tank wall, and successively taking a melted sol out of a separating outlet which separates the melted sol from the unmelted gel. And Japanese Pat. O.P.I. Pub. No. 169743/1982 proposes a method which comprises the steps of tilting a container holding a composition gel, irradiating the surface of the gel with radiowaves from a waveguide to melt the gel to a sol continuously from its surface, and taking the sol out the tilted container by allowing the sol to pass under a microwave shielding plate which advances in accordance with the retrogression of the gel surface.
However, the former requires the processes of taking a composition gel out of a preserving container, cutting the composition gel into pieces, feeding thereof, cleaning melting tanks of respective emulsions and the installation of the same number of melting tanks as that of emulsions used. The latter requires a tilt-setting apparatus and related work as well as a careful shielding work with a microwave shielding plate, in addition, this has an disadvantage of low efficiency attributable to a low melting speed to a sol.
The object of the present invention is to provide a method for melting a photographic composition gel to a sol at a high productivity, without changing the photographic properties.